1. Field of the Invention
The present invention relates generally to digital communications over a communications network. In particular, the present invention is directed toward interfacing a time domain multiplexed (TDM) link with a cell-switched network.
2. Related Art
A communications network serves to transport information among a number of locations. The information is usually presented to the network in the form of time domain electrical signals and can represent any combination of voice, video, or computer data. A typical communications network consists of various physical sites called xe2x80x9cnodesxe2x80x9d, interconnected by conduits called xe2x80x9clinksxe2x80x9d. Each link carries information from one node to another node. End user nodes contain data terminating equipment (DTE) for combining, separating, and transforming data and voice. As such, a DTE can be a voice switch, data switch, or a combination of the two. A series of non-DTE nodes interconnected to each other with ATM links is often referred to as an ATM cloud. A DTE is typically connected to the ATM cloud at one point, and may be connected with an ATM link, other types of data links including frame relay, or digital time domain multiplexed (TDM) communications links such as T1. When an ATM cloud node is connected to a DTE over a non-ATM link, it is typically called circuit emulation.
T1 (also known as DS1) is one type of a TDM communications link. T1 is a synchronous link capable of carrying 24 DS0 channels which are TDM and transmitted over a single physical line. A DS0 channel is a 64 kilobits per second (64 Kbps) channel, which is the world wide standard for digitizing voice conversation. The 64 Kbps bit rate is chosen because an analog voice signal can be adequately represented by a digital data stream if sampled at a rate of 8000 samples per second. If each voice sample is digitized using 8 bits, this results in a digital data stream of 64 Kbps.
A T1 link transmits one T1 frame 8000 times per second (or one frame every 125 xcexcs). Each T1 frame carries a T1 payload with, 24 DS0 timeslots, one for each DS0 channel. Each timeslot contains an 8 bit sample of the corresponding DS0 channel. Each T1 frame also has T1 frame bit that identifies the start of the T1 frame, so that a T1 frame has a total size of 193 bits. This results in a data stream of 1.544 Mbps (8000 frames/secxc2x7193 bits/frame).
A T1 superframe is a group of 12 T1 frames. Each superframe contains a frame bit section composed of 12 frame bits, and a payload section composed of 12 samples for each of the 24 DS0 channels. A T1 extended superframe (ESF) is a group of 24 T1 frames. Each ESF frame is composed of an ESF frame bit section that contains 24 frame bits, and an ESF payload section that contains 24 samples of each of the 24 DS0 channels.
Although T1 was developed for voice communications, it is not limited to voice communications. The physical line can carry digitized voice samples, digital computer data, or any other type of data in any combination in the 24 channels. Thus, a broader definition of a T1 link is a digital transmission link with a capacity of 1.544 Mbps.
Since T1 is a synchronous TDM link, once a channel connection has been setup between two users, that channel is dedicated until the connection is torn down. This channel dedication is an inefficient use of the 1.544 Mbps capacity of a T1 link. For example, assume channel #5 of the 24 T1 channels is set up between user A and user B. Channel #5 will carry all communication between user A and user B. If there is a pause in the communication, (such as user A putting user B on hold) during the transmission of a particular T1 frame, then that particular T1 frame will carry an empty channel #5 timeslot. Even a short pause of one minute can lead to 480,000 T1 frames being transmitted with an empty channel #5 timeslot.
Asynchronous Transfer Mode (ATM) is an asynchronous type of communications protocol based on a cell-switched network. It is designed to be carried over the emerging fiber optical network, called the Synchronous Optical NETwork (SONET), although it can be carried over almost any communications link. The basic unit of ATM is the ATM cell. Each cell contains two parts; a header, which contains routing information, and a payload, which contains the data to be transported from one end node to another.
ATM is considered xe2x80x9casynchronousxe2x80x9d because each node in the network does not know until after a cell arrives where the cell is intended to go. In a synchronous TDM network, each timeslot is assigned a certain time when it is to arrive at each node. The arrival time of a timeslot determines where the timeslot is routed. Thus, the individual frames and timeslots do not need to have routing information within them. The arrival of a particular ATM cell at a node, on the other hand, is not guaranteed to occur at a particular point in time.
ATM is considered a cell-switched network because the node bases its cell routing decision on the information carried in the ATM cell. After a cell arrives, the node reads the destination address in the cell header to determine the destination node. If the current node is not the destination node, the node determines the best route based on network status and the destination address.
There are a number of factors which makes ATM attractive to the telecommunications industry. One is the lower cost of the optical fiber transport mechanism supporting ATM. On a bit per bit basis, a SONET transport mechanism is significantly less expensive than using metallic links by several factors of ten. The theoretical capacity of fiber is in excess of 20 tera bits per second (20 million million bits per second). Current technology is at 40 thousand million bits per second, and will soon increase to 160 thousand million bits per second. As technology improves, more information can be sent over each fiber optic buried in the ground or underwater.
On the other hand, metallic links that can span long distances and are reasonable to manufacture, have long ago reached their theoretical limits of roughly under 500 million bits per second, and are much bulkier than fiber optic links. The metallic link is also susceptible to rust and corrosion, whereas the fiber is relatively chemically inert. Because of signal attenuation (loss of signal strength as a signal travels down a link) on either type of link, repeaters which reamplify the signal are needed. Metallic links attenuate the signals more than do fiber links, so more repeaters for metallic links are needed than for fiber links for a given distance. For instance, a T1 link can span a maximum of just over one mile (6000 feet) before a repeater is needed. It is not unusual for fiber optic links to span 50 to 100 miles between repeaters.
For this reason, it is now cost effective for two end nodes of a T1 link to convert their T1 signals into ATM cells, and send the ATM cells across a SONET network to the destination node. The ATM cells are reconverted to T1 at the destination node. The T1 signals are then sent to a DTE where the DS0 channels are de-multiplexed and sent to their particular user destinations. This approach is referred to as T1 emulation over an ATM network (or T1 over ATM, for short).
T1 emulation requires an interface between the synchronous TDM T1 link and the cell-switched ATM network. Conventional interfaces package at least one complete T1 frame in each ATM cell regardless of the number channels that are currently active on the T1 link. For example, assume that channels #3, #12, and #23 are the only active channels on a T1 link, and the remaining 21 channels are waiting for users to initiate communications. In this case, each T1 frame will have 3 timeslots that carry meaningful digitized samples of channel communications. The remaining 21 timeslots will carry digitized samples of thermal noise. Conventional T1/ATM interface methods load all 24 timeslots from each T1 frame in an ATM cell, which is an inefficient use of the available bandwidth of the ATM network.
A TDM link needs to be interfaced with a cell-switched network so that the available bandwidth of the network is efficiently utilized.
The present invention relates generally to digital communications over a communications network. In particular, the present invention provides a method, apparatus, and computer program product for interfacing a time domain multiplexed (TDM) link to a cell switched network.
In one embodiment of the invention, the TDM link is a T1 link and the cell-switched network is an Asynchronous Transfer Mode (ATM) network. The T1 link supports different combinations of active channel(s) and idle channel(s).
The T1 link is terminated at a first node of the ATM network. The T1 link carries T1 frames that contain a sample of each channel supported by the T1 link in a dedicated timeslot. The idle timeslots are removed from each T1 frame to create a compressed T1 frame, where each idle timeslot carries a sample of an idle channel. One or more compressed T1 frames are inserted in a data cell that travels over the ATM network to a second node, where each compressed T1 frame comprises active timeslots which carry a sample of an active channel.
Data cells are ATM cells that carry compressed T1 frames and are assigned a unique address in their cell header. The address identifies the cell as a data cell and identifies the destination address of the cell. Data cells are distinguished from overhead cells, which carry overhead messages used during interface configuration, by their addresses.
The compressed T1 frames are unloaded from each data cell at the second node. Idle timeslots are inserted in each compressed T1 frame to restore the compressed T1 frames to complete T1 frames. The restored T1 frames are then sent to a DTE. The DTE de-multiplexes the restored T1 frames and sends the channel samples to their respective channel users.
An advantage of the present invention is that samples of idle channels are not carried over the cell-switched network. This results in more efficient utilization of cell-switched network bandwidth than produced by conventional interface methods.
In one embodiment, the first node is a Master node, and the second node is a Slave node. The T1/ATM interface is configured based on a channel status message that is received at the Master node. The channel status message identifies which T1 channels are active channels and which channels are idle channels. A cell structure is determined at the Master node that maximizes the number of compressed T1 frames that can be carried in a data cell. A data address is then assigned to the cell structure.
The cell structure and corresponding data address are communicated to the Slave node in an overhead message carried by one or more overhead cells. The overhead cells are identified by an overhead address in the cell header.
One advantage of the present invention is the ability to adapt to changing channel status. Changing channel status can result in multiple channel status messages, which leads to multiple cell structures. When multiple cell structures are active and a data cell is received, the cell structure of the data cell is identified by the data address in the data cell header.
If a channel status message is received at the Slave node, then the Slave node communicates the channel status message to the Master node in one or more overhead cells. Another advantage of the present invention is that simultaneous changes in cell structure are avoided by requiring all cell structure decisions to be made at the Master node.